None.
The present invention is a motor controller for three-phase DC motors. In particular, the invention relates to programmable pulse width modulated drive for a three-phase DC motor such as a spindle motor used in hard disc drives.
Pulse width modulation techniques are commonly used to drive three-phase DC spindle motors in hard disc drives. The motor controller circuit varies the duty cycle of the driving waveform so that a sensed average motor current matches a current command signal representing a desired motor speed.
If the motor current waveform does not match the torque profile of the motor, the motor will create time varying forces on the spinning disc. This can excite mechanical resonances in the hard disc drive assembly, which can result in audible harmonic tones. The reduction or elimination of audible harmonic tones in disc drives is a concern to disc drive manufacturers. Inexpensive ways to reduce audible harmonic tones continue to be sought.
Driving the three windings of the DC motor with appropriate waveforms can minimize torque ripple. If the torque profile for each stator motor coil is sinusoidal, each motor terminal should be driven with a sinusoidal signal to create a flat torque waveform. A sinusoidal driving current can be accomplished by pulse width modulating the DC source voltage during each of the three phases. Sinusoidal pulse width modulation requires all three windings be driven simultaneously, with one winding being driven high and the other two windings being modulated by driving them high or low or not at all to shape the driving signal. See, for example, T. Kenjo, Electric Motors and Their Controls, Oxford Press, (1991), pp. 134-136.
Many existing motor controller circuits contain a table of wave shaping coefficients that are multiplied by the current demand to determine the drive signal pulse widths which are provided to the motor. Each coefficient corresponds to a particular motor electrical rotational position. Since the three-phase motor is symmetric, each coefficient can be used for a number of positions that are evenly distributed over one full electrical rotation (i.e., 360xc2x0). In some implementations, the wave shaping coefficients are fixed and stored in a read-only memory (ROM). The use of fixed coefficients does not allow for adjustment of the coefficients to match the torque profile of a particular motor design. To address this issue, some motor controller circuits provide for programmable coefficients which are supplied via a serial port and are stored in a register memory of the motor controller. This programmable coefficient approach, however, has several disadvantages. The use of programmable coefficients to tune the spindle motor waveform typically requires 75 or more register bits. This uses scarce register space and die area on the motor controller circuit chip. It also adds tuning complexity for the hard disc drive manufacturer.
The motor controller of the present invention drives a three-phase motor with a waveform that can be varied using a waveform modifier signal which represents only a small number of programmable control bits. A pulse width modulation (PWM) controller provides a pulse width modulated clock signal used to drive the motor. The pulse width modulation clock signal has a duty cycle which is a function of a rotational position of the motor, a magnitude control signal, and the modifier signal.
The PWM controller includes a main lookup table which stores coefficients as a function of an electrical rotational position of the motor. The main lookup table coefficients will control the duty cycle of the pulse width modulated clock signal to provide a basic waveform such as, for example, a sinusoidal drive waveform.
The PWM controller also includes a modifier lookup table which contains modifier coefficients as a function of electrical rotational position. When the modifier coefficients are combined with the main coefficients, the result is a waveform of a second, different shape, such as a trapezoidal waveform shape.
The modifier signal selects the extent to which the modifier coefficient is used to modify the main coefficient. By selectively varying the combination of the modifier coefficients with the main coefficients, a family of different drive waveforms can be produced.